AIRFLOW REQUIREMENTS

Naturally aspirated engines with throttle bodies

rely on atmospheric pressure to push an airfuel mixture into the combustion chamber vacuum created by the downstroke of a piston. The mixture is then compressed before ignition to increase the force of the burning, expanding gases. The greater the mixture compression, the greater the power resulting from combustion AIRFLOW REQUIREMENTS Engine Compression Higher compression increases the thermal efficiency of the engine because it raises compression temperatures, resulting in hotter, more complete combustion.

SUPERCHARGING PRINCIPLES The amount of force an airfuel charge produces when it is ignited is largely a function of the charge density. Density is the mass of a substance in a given amount of space.

FIGURE 19-3 The more air and fuel that can be packed in a cylinder, the greater the density of the airfuel charge. SUPERCHARGING PRINCIPLES An engine that uses atmospheric pressure for intake is called a naturally (normally) aspirated engine. Another way to achieve an increase in mixture compression is called supercharging. This method uses a pump to pack a denser airfuel charge into the cylinders. Since the density of the airfuel charge is greater, so is its weight and power is directly related to the weight of an airfuel charge consumed within a given time period.

SUPERCHARGING PRINCIPLES In addition to the increased power resulting from combustion, there are several other advantages of supercharging an engine including: It increases the airfuel charge density to provide highcompression pressure when power is required, but allows the engine to run on lower pressures when additional power is not required.

SUPERCHARGING PRINCIPLES The pumped air pushes the remaining exhaust from the combustion chamber during intake and exhaust valve overlap. The forced airflow and removal of hot exhaust gases lowers the temperature of the cylinder head, pistons, and valves, and helps extend the life of the engine.

SUPERCHARGERS A supercharger is an engine-driven air pump that supplies more than the normal amount of air into the intake manifold and boosts engine torque and power. A supercharger provides an instantaneous increase in power without the delay or lag often associated with turbochargers. In basic concept, a supercharger is nothing more than an air pump mechanically driven by the engine itself.

TURBOCHARGERS The major disadvantage of a supercharger is its reliance on engine power to drive the unit. By connecting a centrifugal supercharger to a turbine drive wheel and installing it in the exhaust path, the lost engine horsepower is regained to perform other work and the combustion heat energy lost in the engine exhaust (as much as 40% to 50%) can be harnessed to do useful work.

TURBOCHARGERS The turbochargers main advantage over a mechanically driven supercharger is that the turbocharger does not drain power from the engine. In a naturally aspirated engine, about half of the heat energy contained in the fuel goes out the exhaust system.

FIGURE 19-7 A turbocharger uses some of the heat energy that would normally be wasted. TURBOCHARGERS FIGURE 19-8 A turbine wheel is turned by the expanding exhaust gases. TURBOCHARGERS A turbocharger consists of two chambers connected by a center housing. The two chambers contain a turbine wheel and a compressor wheel connected by a shaft which passes through the center housing.

TURBOCHARGERS As exhaust gas enters the turbocharger, it rotates the turbine blades. The turbine wheel and compressor wheel are on the same shaft so that they turn at the same speed. Rotation of the compressor wheel draws air in through a central inlet and centrifugal force pumps it through an outlet at the edge of the housing.